LB leukemia is a nonimmunogenic T cell tumor which spontaneously arose in a BALB/c mouse; efforts to induce immunological rejection of the leukemic cells have always failed. The leukemic cells grow rapidly and progressively in the syngeneic host invading spleen, lymph nodes and liver. A cell line (LBC) was developed from the original tumor. Both the original tumor and the cell line have been characterized as expressing the Thy 1+, CD3-, CD25+, MHC class I+, class II-, CD4- (original tumor), CD4+ (cell line), CD8+, gp70-, J11d.2+ phenotypes. Immunization of syngeneic mice with irradiated LBC cells induced cytotoxic T lymphocytes as well as anti-LBC antibodies which reacted with components of 14, 16 and 27 kDa present on LB tumor cells, LBC cell line and normal thymocytes but not on normal lymph node cells. Immunization of syngeneic mice with LBC cells partially protected them against subsequent challenge with the original tumor cells. The effect of sera from tumor-bearing mice and the super-natants from short term cultures were studied on cell proliferation. An inhibitory activity was demonstrated in these fluids, which was abrogated by addition of exogenous IL-2. ELISA showed the presence of soluble IL-2R alpha chain both in the conditioned medium as well as in the serum, which was demonstrated to be responsible for the inhibitory activity. The soluble IL-2R was produced by LB leukemic cells and exerted the inhibitory activity blocking cell proliferation and modulating immune response by binding to free IL-2. Using reverse-transcription PCR, mRNA for IL-2 was found to be present in tumor cells. Our findings indicate that LB cell proliferation is mediated by an autocrine pathway involving endogenous IL-2 generation, despite the fact that these cells are not dependent on exogenous IL-2 to grow in culture. The relationship between tumorigenicity and expression of MHC class II was also investigated. In vitro treatment with IFN-gamma failed to induce the expression of class II antigens in LBC cell line. Therefore these cells were tri-transfected by a liposome-mediated protocol with 1-A alpha d, I-A beta d genes and pSV2neo. Cells were selected to grow in medium containing Genetecin (G418) and surviving transfectants were cloned. Three I-A+ clones were obtained (LBCT) and were used to induce a specific CTL response against tumor cells. Syngeneic mice inoculated with 10(3) LBCT cells failed to develop a tumor while the DT50 of mice injected with 10(6) LBCT cells was three times the value for mice injected with LBC cells (I-A-). It is suggested that neoexpression of MHC class II molecules enhances anti-tumor response by transforming tumor cells into professional antigen-presenting cells, which may be used to improve tumor-specific immunity in the autologous host.